<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>两列布局</title>
</head>
<style>
  /* 浮动 */
  .box{
    height: 600px;
  }
  #left{
    width: 200px;
    background: rgb(179, 15, 15);
    float: left;
  }

  #right{
    background: rgb(10, 106, 144);
    margin-left: 330px;
  }

  /* 定位 */
  #div3 {
      /* width: 200px; */
      height: 200px;
      background: #ccc;
      position: absolute;
      left: 0;
      right: 0;
    }

</style>
<body>
  <div class="box" id="left"></div>
  <div class="box" id="right"></div>
  
  <div id="div3"></div>
  
</body>
</html>